1. Field of the Invention
This invention pertains generally to saturable absorbers, and more particularly to an HCG-based saturable absorber and its applications.
2. Description of Related Art
A semiconductor saturable absorber mirror (SESAM) is a powerful device for generating femtosecond pulses in a mode-locked laser. Traditional SESAMs are built on top of a distributed Bragg reflector (DBR) structure which provides a broad reflection band. A SESAM is a mirror structure with an incorporated saturable absorber which is made using semiconductor fabrication technology. Typically, a SESAM contains a semiconductor Bragg mirror with quantum well absorber layers near the surface. The materials of the Bragg mirror within a SESAM have a larger bandgap energy, whereby essentially no absorption occurs in that region, and these devices are also referred to as saturable Bragg reflectors (SBRs). To obtain a large modulation depth, such as required for passive Q switching, a thicker absorber layer can be utilized, while a suitable passivation layer can be added over the top surface to increase device lifetime.
The absorption material often comprises bulk material, quantum wells (QWs) or quantum dots, with Gallium Arsenide (GaAs) and Aluminum Arsenide (AlAs) being typically utilized for the DBR structure. In response to the low index contrast between these two materials, a large number of pairs of material are required to provide a large high reflection bandwidth.
One of the limitations of SESAM is found in its saturation fluence, typically on the order of tens to hundreds μJ/cm2. As the repetition frequency of the mode-lock laser increases, the energy for each pulse decreases, requiring a lower saturation fluence to mode lock the laser. Several structures based on DBR solve this problem, however, this resolution is provided at the expense of reduced bandwidth.
Accordingly, there is a need of saturable absorbers having a high bandwidth. Saturable absorbers according to the present invention fulfill that need and provide lowered saturation fluence, and other beneficial attributes suitable to a number of applications.